MSE Walls Become the Central Aesthetic Feature at La Villita Apartment & Homes

MSE Walls Become the Central Aesthetic Feature at La Villita Apartment & Homes

Supporting a Variety of Loading Conditions
Rusticated, stucco-textured top panels above normal water elevation
Irving, TX
Las Colinas Land Limited Partnership
Cousins Properties, Inc.
Carter & Burgess

Responding to specialized customer needs with beauty, performance, service life and economy – that perfectly describes Reinforced Earth® custom-engineered Mechanically Stabilized Earth (MSE) retaining walls from The Reinforced Earth Company (RECo). But sometimes the architecture drives the project and the engineering must follow. That was exactly what Cousins Properties, Inc., manager for land owner Las Colinas Land Limited Partnership, found as it began developing the La Villita luxury apartments and homes, in Irving, TX. And Cousins Properties’ customers indirectly drove the aesthetic design of La Villita through their expectation of an architectural character that would be commensurate with luxury living in Las Colinas.

Project consultant Carter & Burgess (Ft. Worth, now part of Jacobs Engineering Group, Inc.) needed to create an appropriate aesthetic feature, meaning La Villita homes and apartments would surround a lake and meandering waterways. The homes had to be protected from both the normal water level and periodic flooding of the lake and waterways, which ultimately flow into the Elm Fork of the Trinity River, requiring installation of retaining walls along the waterways. These walls became central aesthetic features of the community, creating the need for a low key but distinctive look, while also offering reliable performance, long service life and, of course, initial economy.

In addressing the aesthetic concept created by Carter & Burgess’ in-house landscape architects, RECo identified an existing nearby highway project having an appearance similar to and adaptable to what was desired. The Dallas Central Expressway featured more than 2 million sq. ft. of wide rectangular facing panels set off by tall, narrow facing panels, giving the appearance of pilasters separating sections of the walls. Adapting this concept to La Villita, RECo designed walls typically 13 ft. tall, comprising two stacked rectangular facing panels, topped by precast coping and arranged in 4 panel wide (60 lf.) bays, with narrow pilaster panels delineating the bays.

The lower panels had smooth faces, while upper panels had a central rectangular inset where the concrete surface had a stucco form liner finish, adding to the distinctive appearance of the completed walls and the La Villita community (Fig. 1).

While the geotechnical design requirements were established by Rone Engineering (Dallas) the scope of RECo’s design included the analysis of a variety of loading conditions. Although the pedestrian promenade along the walls imposed little load, the MSE design was required to consider the influence of building surcharge loading which began 26 feet back from the wall face. The walls also had to be designed for three different conditions related to the waterways – dry during construction (Fig. 2), saturated at normal pool conditions (water at mid-height of the wall), and 100-year flood, with water within 1 ft. of the top of wall. The design also had to address a 3 ft. drawdown condition, in which the water level in front of the wall is assumed to be 3 feet below the backfill saturation level, producing an unbalanced hydrostatic head. This saturation/drawdown condition was evaluated at all elevations below the 100 year flood level to ensure wall stability under all conditions.

The addition of hydrostatic pressure to the design reduces both the pullout safety factor of the soil reinforcements and the sliding safety factor of the MSE structure. Responding to both insufficient pullout resistance and insufficient sliding resistance, the reinforcement length of the La Villita Reinforced Earth walls was increased. As a result, reinforcing strip length in the compacted granular backfill behind the walls was generally 13 ft., equal to 100% of the design height ("100% aspect ratio"), as compared to the "standard design" 70% aspect ratio in structures not subject to flooding.

Both facing and pilaster panels had standard Reinforced Earth tie strips cast into their back faces and bolted to the Reinforced Earth high adherence ribbed reinforcing strips. A woven filter cloth covered all panel and pilaster joints on the backfill side, to prevent escape of backfill during post-flood drainage, and a pedestrian railing was attached to the coping and supported by a cast-in-place moment slab. As part of the overall aesthetic character of the community, a number of planting wells were located approximately 10 ft. behind the wall. Not only did Carter & Burgess specify a commercial root barrier system to a depth of 3 feet to contain the plantable soil and prevent root intrusion through the wall facings, but they also specified a supplemental lateral force on the walls as a result of this root system. To accommodate the plantings, RECo designed the upper reinforcing strips to be gradually deflected downward to go below the planting wells.

Construction of the La Villita Reinforced Earth walls was closely linked to overall construction of the project’s water features, which began as a natural channel and a small pond. Recognizing the aesthetic appeal of water near residences, the owner enlarged and deepened the pond to become an 85 ft. deep lake, with a flow control weir system at its downstream outlet. Initial filling of the lake was to a point 5 ft. below the Reinforced Earth walls, allowing construction in the dry. Upon successful completion of wall erection by Coppel Construction Co., the flow control weir structure was used to raise the water level in the lake and canal by 10 ft., to the mid-height of the walls (normal pool).

The La Villita project required a very special architectural look as an integral part of the value of the property. At the same time, Cousins Properties sought to build the project at the lowest cost consistent with that aesthetic character. Reinforced Earth walls, well-known for their structural capabilities and cost effectiveness, would not normally have been considered to produce the look required at La Villita but, due to RECo’s imagination and "we will figure out a way" spirit, they were both considered and utilized. Though many special architectural challenges and design considerations had to be addressed, the result was a positive experience and an end product in which everyone takes pride. The engineering sophistication, long-term performance and initial economy of the Reinforced Earth walls may not be evident to the residents of La Villita, but they cannot miss the beauty of their own neighborhood Reinforced Earth walls.